[RFC PATCH 3/3] mm/memcg: Allow the task_obj optimization only on non-PREEMPTIBLE kernels.

Sebastian Andrzej Siewior
2021-12-22T11:41:11Z

Based on my understanding the optimisation with task_obj for in_task()
mask sense on non-PREEMPTIBLE kernels because preempt_disable()/enable()
is optimized away. This could be then restricted to !CONFIG_PREEMPTION kernel
instead to only PREEMPT_RT.
With CONFIG_PREEMPT_DYNAMIC a non-PREEMPTIBLE kernel can also be
configured but these kernels always have preempt_disable()/enable()
present so it probably makes no sense here for the optimisation.

Restrict the optimisation to !CONFIG_PREEMPTION kernels.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
mm/memcontrol.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1e76f26be2c15..92180f1aa9edc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2126,7 +2126,7 @@ struct memcg_stock_pcp {
local_lock_t stock_lock;
struct mem_cgroup *cached; /* this never be root cgroup */
unsigned int nr_pages;
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
/* Protects only task_obj */
local_lock_t task_obj_lock;
struct obj_stock task_obj;
@@ -2139,7 +2139,7 @@ struct memcg_stock_pcp {
};
static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
.stock_lock = INIT_LOCAL_LOCK(stock_lock),
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
.task_obj_lock = INIT_LOCAL_LOCK(task_obj_lock),
#endif
};
@@ -2228,7 +2228,7 @@ static void drain_local_stock(struct work_struct *dummy)
* drain_stock races is that we always operate on local CPU stock
* here with IRQ disabled
*/
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
local_lock(&memcg_stock.task_obj_lock);
old = drain_obj_stock(&this_cpu_ptr(&memcg_stock)->task_obj, NULL);
local_unlock(&memcg_stock.task_obj_lock);
@@ -2837,7 +2837,7 @@ static inline struct obj_stock *get_obj_stock(unsigned long *pflags,
{
struct memcg_stock_pcp *stock;

-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
if (likely(in_task())) {
*pflags = 0UL;
*stock_pcp = NULL;
@@ -2855,7 +2855,7 @@ static inline struct obj_stock *get_obj_stock(unsigned long *pflags,
static inline void put_obj_stock(unsigned long flags,
struct memcg_stock_pcp *stock_pcp)
{
-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
if (likely(!stock_pcp))
local_unlock(&memcg_stock.task_obj_lock);
else
@@ -3267,7 +3267,7 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
{
struct mem_cgroup *memcg;

-#ifndef CONFIG_PREEMPT_RT
+#ifndef CONFIG_PREEMPTION
if (in_task() && stock->task_obj.cached_objcg) {
memcg = obj_cgroup_memcg(stock->task_obj.cached_objcg);
if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
--
2.34.1